The present invention is in the field of electronic reproduction technology and is directed to a method for optimizing frequency-modulated rasters using threshold hills.
Different half-tone values (gray scale values) of an image to be reproduced can be produced in the print only on the basis of a surface modulation in that the gray scale values are resolved into highly resolved binary values having only two brightness values (black and white).
In autotype rastering (dot screening), the surface modulation ensues in that the gray scale values of the image are converted into picture elements of different sizes and are printed, whereby the picture elements are arranged in a regular raster structure having periodically repeating raster meshes. In order to minimize disturbing Moire patterns, the dot rasters of the individual inks are arranged at different screen angles.
German reference 2,827,596 discloses an autotype rastering method for generating dot rasters having arbitrary screen angling using a threshold hill. In this rastering method, a matrix is subdivided into a plurality of matrix elements and a threshold is allocated to every matrix element. The totality of thresholds, referred to as a threshold hill or raster hill, represents the repeating basic structure for every raster mesh of the dot raster.
The recording material for the rastered image is divided into a plurality of surface elements. When recording the rastered image in an output device (recorder), the picture elements in the individual raster meshes of the dot raster grid are composed of exposed surface elements (illumination pixels; device pixels). The check to determine whether or not a device pixel is to be illuminated or exposed as part of a picture element ensues on the basis of a comparison of the corresponding gray scale values of the image to the thresholds of the threshold hill. The results of the comparison are retained as bits in what is referred to as a bit map wherein every set bit corresponds to an exposed surface element or device pixel (or vice versa in the case of a negative presentation). The bit map generally has the size of the recording surface. The control of the exposure beam in the recorder ensues with reference to the bit map.
FIG. 1 shows an example of a threshold hill for an autotype rastering. In this example a two-dimensional threshold matrix 1 has 5.times.5 matrix elements 2 that are filled with the thresholds 1 through 255, namely for that case wherein the individual gray scale values of the image are represented by numbers between 0 and 255, whereby the number 0 corresponds to "black" and the number 255 corresponds to "white". The threshold matrix 1 generally has the size of a raster mesh of the dot raster. In order, for example, to produce the gray scale value 192 (25% surface coverage), a check is carried out for every matrix element 2 within the threshold matrix 1 to determine if the gray scale value 192 is lower than the threshold allocated to the matrix element 2. When this is the case, the appertaining bit is set in the bit map and the corresponding device pixel is blackened; otherwise, the corresponding bit is not set.
FIG. 2 shows a pixel within a raster mesh 3, said pixel having been produced according to an autotype rastering method. The pixel is composed of seven exposed device pixels 4 of 25 possible device pixels 4 within a raster mesh 3 and thus represents a surface coverage of approximately 25% or, respectively, the gray scale value 192. Typically, the thresholds are assigned such that the set bits in the bit map or the exposed device pixels form a continuous, growing, black area (pixel, printing dot) given an increasing gray scale value.
Since the threshold matrix 1 for the threshold hill is noticeably smaller than the bit map, the allocation of the bits of the bit map to the thresholds of the threshold matrix 1 is continuously repeated in the x-direction and in the y-direction of the bit map, this being shown in FIG. 3.
FIG. 3 shows the periodic allocation of the thresholds of the raster meshes 3 or of the individual raster meshes 3 of a dot raster to a bit map 5. A few pixels 6 having a surface coverage of 25% are indicated in the raster meshes 3 respectively composed of 25.times.25 device pixels 4. Since the allocation of the threshold matrix 1 in the x-direction and the y-direction is continuously repeated, this leads to the formation of a grid structure.
Alternatively to autotype rastering, the surface modulation of the inks can also ensue according to a method for frequency-modulated rastering, for example according to the method set forth in the German reference 2,931,098. In frequency-modulated rastering, the gray scale values of the image are reproduced by an arrangement of small printing dots (device pixels) of the same size that are more or less arbitrarily selected in the recording surface. The plurality of device pixels per surface unit defines the gray scale value that is reproduced.
Various methods, for example error diffusion, ordered dither, etc., are known for frequency-modulated rastering. All methods are based thereon that the status of the neighboring pixel is taken into consideration with a certain weighting in the decision as to whether or not a device pixel is to be set. Further, an error propagation from one device pixel to the next is also undertaken, i.e. the status of the most recently set device pixel has influence on the next pixel to be processed.
FIG. 4 shows an example of a frequency-modulated rastering. The set bits or the corresponding device pixels 4 are more or less arbitrarily distributed within the surface unit in the bit map 5 and thus no longer form a continuous area. A surface coverage of 25% is shown.
The advantage of an autotype rastering method with a threshold hill is based on the rastering being carried out very quickly both in software as well as in hardware. In traditional, frequency-modulated rastering methods, a plurality of time-consuming calculation operations are required for every device pixel. This has a disadvantageous effect on the operating speed.
It is also known to combine an autotype rastering method and a frequency-modulated rastering method with one another and to fill a threshold hill with an arbitrary sequence of thresholds that do not form a contiguous area. To this end, FIG. 5 shows a threshold matrix 1 having a random sequence of 5.times.5 thresholds for the frequency-modulated rastering and FIG. 6 shows a pixel of seven randomly distributed, exposed device pixels 4 within a surface unit 7 that is produced by a frequency-modulated rastering. This procedure, however, has the disadvantage that disturbing residual structures result in color areas having constant tonal value even given the most dexterous selection of the random distribution of the thresholds. Since the threshold hill periodically repeats in the x-direction and in the y-direction, these residual structures are also repeated over the entire recording surface. Since the eye is especially sensitive to such residual structures, they are not tolerable in practice.